How often are you doing water changes and what are your tank parameters? It is possible it's just skin irritation from your water or from going into water that's different that what it's used to. Of course it's possible it's some sort of parasite like gill flukes. Keep your water very clean, and keep an eye on it. Is the fish appearing to be dark? I'd hate to see you throw meds at it not really knowing what's wrong, that fish is already stressed enough. Any chance you can take a pic and post it?